The invention relates to a device and a method for monitoring and calibrating a device for measuring the profile depth of a tyre, in particular of a motor vehicle tyre.
In order to measure the profile depth of tyres, different methods for manually or automatically measuring the profile depth are known, which comprise methods for measuring in a mechanical or non-contact manner as well as methods for measuring a stationary or rolling vehicle or, respectively, tyre.
Triangulation methods using laser scanners for measuring profile depth on a roller chassis dynamometer are described, for example, in the German patent specification DE 197 05 047 A1 and the European patent specification EP 1 394 503 B1.
The German patent specification DE 10 2006 062 447 A1 likewise measures a region of the tyre profile while the tyre rolls over an opening by means of a light section triangulation method. The method provides that an illumination line is guided with the aid of a prism that rotates at a high speed transversely to the rolling direction of the wheel across the profile surface. The entire section of the tyre can be scanned sequentially and with high density by means of the illumination line that is displaced in a parallel manner, while the tyre surface located on the opening is situated in the resting state relative to the measuring device.
The European patent specifications EP 0 469 948 B1 and EP 1 952 092 B1 as well as the American patent specification U.S. Pat. No. 7,578,180 B2 describe further variants of triangulation methods for measuring the profile of the tread of a tyre while the tyre traverses an opening.
The German patent specification DE 10 2012 224 260 describes a device for measuring the profile of the running surface of a tyre, said device comprising a plurality of measuring modules that are disposed adjacent to one another and transversely to the rolling direction of the wheel and comprise in each case an illumination apparatus and an image capture apparatus.
In several embodiments, the illumination apparatuses are designed such that they project a pattern, which comprises a plurality of light lines, onto the profile of the tyre. The illumination apparatuses can thereby be particularly designed in such a manner that the individual light lines of the pattern are spatially coded so that said light lines can be unambiguously identified. Such an unambiguous identifiability of the light lines is necessary in order to achieve a high degree of measuring accuracy in connection with a large number of light lines. A spatial coding can, e.g., be implemented by means of a suitably designed diffractive optical element which generates a line pattern which has a spacing that varies between lines. As an alternative, the width of the lines, the line pattern and or the shape of the lines can also be varied.
The application of such profile depth measuring instruments, which are based on a triangulation method, in, for example, entrances to gas stations and the like requires continuous use 24 hours a day in harsh environmental conditions which include, e.g., such things as vibrations and changing temperatures.
As a result of the environmental factors, the mechanical suspension of the illumination apparatuses and image capture apparatuses as well as possibly of further optical elements for beam deflection, such as, e.g., mirrors or prisms, is heavily stressed, so that the factory-provided calibration can no longer be seen as valid in certain circumstances. In order to ensure the constructive and mechanical long-term stability of the optical elements, which are used for highly precise measuring tasks, against mutual tilting, twisting, distortion and displacement, enormous demands are placed on the material, statics and the constructive design of the mounting suspensions and connections.
It is the aim of the invention to provide an improved device and an improved method for the optical measurement of the profile depth of a tyre, in particular the tyre of a motor vehicle, which enable a high quality of measurement to be provided with the lowest possible design complexity and with the lowest possible maintenance requirements over a service life that is as long as possible.